What is Endcapping?

When we functionalize our silica it is impossible to react with all available silanol groups (free OH groups on the silica surface) so we "endcap" the residual ones with a capping agent such as trimethylsilylchloride to form Si-O-Si(CH3)3.

There are two main benefits of end capping:

First, it renders the silica non-acidic and non-polar. Bare silica gel for chromatography with all its free silanols is polar and somewhat acidic (pKa~5). In chromatography this polar interaction is the basis for the separation, but for functionalized silica that is used as a scavenger or a reagent, the only interaction that should occur is the one with the grafted function and not a non-specific binding with the surface.

The second reason is that when functionalized silica is used in harsh conditions, the end-capping prevents the surface from being attacked and destroyed. The figure below represents what the surface structure looks like.